Spray modeling improves engine combustion control and simulation

May 26, 2011 // By Rolf Reitz, Reaction Design
In order to achieve highly efficient internal combustion engines, designers either need to employ extensive test series or they have to rely on combustion simulation. The first method is expensive and time-consuming; the second hitherto was very inexact. New modeling approaches yield exact results even with relatively small computers.

In response to environmental and energy security concerns, the Obama administration has recently announced government regulations imposing stricter pollutant emissions mandates as well as higher fuel-efficiency standards for both automobiles and trucks.

One impact is the rising cost of aftertreatment systems in long-haul trucks to clean up pollution generated by the engine. For example, selective catalytic reduction systems, which remove nitric oxides from heavy-duty diesel-truck exhaust, can add as much as $20,000 to the price of a truck. This is about the same cost as the engine itself. Such systems furthermore require maintaining a second tank, typically containing urea, which introduces additional operating and maintenance costs for the end-user. A better option would be to design engines that solve the pollution problem in the cylinder.

To combat the growing costs, the auto industry is trying to address the issues at the source, through cleaner and more efficient engine design and control. Novel engine designs offer the promise of achieving revolutionary improvements in efficiency while maintaining low emissions through careful application of fuel kinetics.

Designs such as Reactivity Controlled Compression Ignition (RCCI) use a careful application of dual fuels to control the condition under which fuel is ignited and burned, offering substantial improvements in efficiency and emissions. Technologies such as RCCI require precise introduction of spray droplets for liquid fuel and air to be rapidly mixed in carefully controlled levels. Defining the optimal atomizer design, injection timing, and combustion chamber design is a complicated process where many variables have to be investigated.

The design problem gets even more difficult when you consider that the fuel itself has become a variable with different atomization and combustion characteristics. Using engine testing alone to achieve design optimization will result in unacceptably high development costs and time-to-market for these needed technologies. More accurate simulations of the spray and combustion phenomena inside the engine are key to achieving these goals quickly and efficiently. Engine modeling can

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